Honing oil serves two purposes when sharpening with oilstones. The first is general lubrication: it makes the blade slide more easily. The second use is to make the steel particles float away in the oil. This prevents the oilstone from clogging. This is very important on all oilstones, but particularly important on artificial India stones, which are porous. You can make your own honing oil using a 50-50 mixture of mineral oil and kerosene. This works fine, but many people prefer Norton's honing oil. Aside from having the perfect viscosity, Norton's honing oil has one great feature to recommend it: it's made of highly refined mineral oil so it's safe for use on kitchen utensils and also doesn't smell as harshly industrial as the mineral oil/kerosene mix.